1. Field of the Invention
This invention relates to an image display method and an image display apparatus for displaying a moving image of an object of examination such as a coronary artery for determining an optimum direction of observation of the object by a medical X-ray apparatus when reading the moving image of the object that can be acquired by means of a medical equipment such as an X-ray diagnostic apparatus for intravascular surgery (intervention).
2. Description of the Related Art
Objects of examination of human bodies include coronary arteries. Coronary arteries surround the heart as blood vessels to supply blood to the muscles of the heart. A serious condition arises when any of the coronary arteries falls into a morbid state such as stenosis. An intravascular intervention using an X-ray diagnostic apparatus is brought in to cure such a disease.
An intravascular intervention using an X-ray diagnostic apparatus is a risky manual operation of driving a surgery device into a coronary artery. For the intervention, the detector of the X-ray diagnostic apparatus is rigidly secured in position and X-rays are continuously irradiated from a selected view angle, while the operator proceeds with the manipulation, seeing the image of the blood vessel being displayed on the monitor screen as object of examination. The manipulation can be facilitated when the operator can see an image of the blood vessel taken from an optimal view angle.
FIGS. 28 and 29 of the accompanying drawings show how a part of a human body appears in tow different view angles. The view angles include view angle θ illustrated in FIG. 28 and view angle ø illustrated in FIG. 29. By turn, the view angle θ is expressed in terms of a head direction (CRA) and a tail direction (CAU) relative to the object of examination 1 of a human body, whereas the view angle ø is expressed in terms of a first tilt direction (RAO) and a second tilt direction (LAO) relative to the object of examination 1.
However, when the view angle is shifted from an optimal view angle during the intervention, the coronary artery in question may be overlapped with other blood vessel or a branching blood vessel may be overlapped with the coronary artery to consequently give rise to a phenomenon of foreshortening, where the coronary artery appears shorter than its proper length. Such a phenomenon makes the manipulation of the intervention difficult. Therefore, it is important to find out an optimal view angle from the information acquired before the intervention. In the case of a coronary artery, foreshortening is a phenomenon that makes the length of the coronary artery projected onto a two-dimensional plane appear shorter than the length of the coronary artery actually running in a three-dimensional space. Foreshortening 0% refers to that the actual length of the coronary artery is equal to the apparent length thereof and foreshortening 100% refers to that the coronary artery appears simply as a spot.
There is a technique of determining an optimal view angle for an intervention. With this technique, a technology of three-dimensionally displaying a rotating coronary artery (coronary 3D, coronary tree) on a monitor screen on the basis of X-ray images in two directions is employed to determine an optimal view angle. This technique has become popular for clinical applications.
FIG. 30 of the accompanying drawings schematically illustrates how a three-dimensional image is structured by epipolar geometry. For example, let us refer to the projected image of coronary artery 1a of a human body picked up by imaging the coronary artery from a frontal direction as Frontal Image 2 and the projected image picked up by imaging the coronary artery from a direction different from the frontal direction, or a lateral direction, as Lateral Image 3. The part of the coronary artery 1a projected on point A on the Lateral Image 3 is found somewhere on line B but cannot be identified. The line B is projected onto line C on the Frontal Image 2. The coronary artery 1a is projected somewhere on the line C. Thus, as the operator of the intervention manually specifies the corresponding point on the Frontal Image 2, the position of the coronary artery 1a in a three-dimensional space is defined. In other words, it is necessary to specify the coordinates of the point on the Frontal Image 2 and those of the corresponding point on the Lateral Image 3 in order to identify the three-dimensional position of the coronary artery 1a. 
FIG. 31 schematically illustrates how a rotating blood vessel is displayed three-dimensionally on a monitor screen. The rotating blood vessel is displayed three-dimensionally as a moving image that is obtained when the coronary artery 1a is observed in the first tilt direction (RAO) and the second tilt direction (LAO) and the view angle (point of view) is shifted. Each of the arrows in the three-dimensional images of the blood vessel indicates the rotating/moving direction of the coronary artery 1a. 
More specifically, a moving image that is obtained when the view angle is continuously shifted with time to RAO=50°, RAO=90°, RAO=130°, . . . , LAO=10° is displayed on a monitor screen. Only the view angles of RAO and LAO of RAO=50°, RAO=90°, RAO=130°, . . . , LAO=10° are shown for the purpose of simplicity. The moving image that is obtained as the view angle is shifted relative to the coronary artery 1a appears as a rotating image of the coronary artery 1a. Thus, an optimal view angle can be determined from the rotating image of the coronary artery 1a being displayed on the monitor screen.
An optimal view angle is such that, for example, (a) an angle that makes the morbid part, e.g., a stenotic part, appear longest, which is an angle that makes branched blood vessels, if any, appear wide open, (b) an angle that makes the morbid part, e.g., a stenotic part, appear narrowest, (c) an angle that makes the morbid part, e.g., a stenotic part, free from any other overlapping blood vessel or (d) an angle that makes the morbid part appear moving least.
Blood vessels change their positions as the heart beats. An optimal view angle is determined for a blood vessel so as to structure a three-dimensional blood vessel image in a cardiac phase at a moment of the moving blood vessel and satisfy above (a), (b), (c). However, the determined optimal view angle cannot necessarily be optimal in all the cardiac phases. In other words, if such a view angle is optimal in a cardiac phase, it may not necessarily be so in other cardiac phases and hence the requirements of above (a), (b), (c) may not necessarily be satisfied in other cardiac phases. Additionally, it is not possible to determine an optimal view angle in terms of (d) an angle that makes the morbid part appear moving least. For example, if any other blood vessel does not overlap the coronary artery 1a in a cardiac phase, the coronary artery 1a is more often than not hidden by some other blood vessel to make an intervention difficult.
A heat phase can be explained by means of an electrocardiogram signal as follows. An electrocardiogram (ECG) is obtained by sensing the movement of the heart as an electrical signal. As shown in FIG. 32 of the accompanying drawings, an R-wave appears when the ventricle begins contraction. A cardiac phase shows a temporal phase of the electrocardiogram signal E in the time interval from an R-wave to the next R-wave. Generally, an R-wave is defined as 0% temporal phase and the next R-wave is defined as 100% temporal phase and the temporal phase at any moment in the time interval is determined by means of the temporal ratio of the moment. For example, an end-diastolic phase is expressed to be at or near 75% temporal phase.
As pointed out above, the determined optimal view angle cannot necessarily be optimal in all the cardiac phases. To solve this problem, a technique of preparing four-dimensional data of (x, y, z, t) including three-dimensional spatial image data of (x, y, z) and temporal elements (t) and determining an optimal view angle for all the cardiac phases by means of these four-dimensional data may be conceivable.
However, the data obtained by means of such a technique include both information acquired when the view angle is shifted and information on the movement of the heart. Then, the image displayed on the monitor screen is a three-dimensional image of a blood vessel that is obtained when the view angle is shifted to which the heartbeat movement is added. When both the shift of the view angle and the heartbeat movement are displayed on the monitor screen, the movement of the coronary artery 1a shown on the monitor screen becomes complicated to make it difficult to determine an optimal view angle. Determine separately an optimal view angle by shifting the view angle and also an optimal view angle in a cardiac phase results in determining only a locally optimal view angle to make it very time consuming to determine an optimal view angle for all the cardiac phases. Known documents relating to this technical field include U.S. Pat. No. 6,501,848.
The present invention provides an image display method and an image display apparatus that can display an image for determining an optimal view angle in a short period of time.